A fuel cell or an air cell is an electrochemical energy device which uses as an oxidant, for example, the oxygen in the air, and takes out as electrical energy the energy generated by the chemical reaction between a compound to be a fuel and a negative electrode active material. A fuel cell or an air cell has an higher theoretical energy capacity than the theoretical energy capacities of secondary batteries such as Li-ion batteries, and can be used as in-vehicle electric power supplies, stationary distributed electric power supplies at homes and factories, or electric power supplies for portable electronic devices.
On the oxygen electrode side in a fuel cell or an air cell, an electrochemical reaction reducing oxygen occurs. The oxygen reduction reaction hardly proceeds at relatively low temperatures, and in general, the reaction can be promoted by a noble metal catalyst such as platinum (Pt). However, the energy conversion efficiency of fuel cells or air cells is still not sufficient. The oxygen reduction reaction occurs in a high electric potential region, and hence even a noble metal such as Pt is degraded by dissolution, to lead to a problem concerning the securement of long-term stability and reliability. In addition, a catalyst mainly composed of a noble metal such as Pt is expensive, to boost the price of the whole system of a fuel cell or an air cell to limit the widespread use of the cell concerned. Accordingly, the development of an inexpensive catalyst not using a noble metal such as platinum and having a high oxygen reduction ability is demanded.
As a catalyst not containing Pt, there have been known, for example, organometallic complexes, carbon nitride, transition metal chalcogenides, transition metal carbides and transition metal nitrides; however, any of these is insufficient with respect to catalytic activity or durability not to produce performance beyond the performance of the Pt-based catalysts.
Non Patent Literature 1 and Non Patent Literature 2 disclose that among these, part of the oxides of the group 4 and group 5 elements of transition metals have activity for oxygen reduction reaction. In addition, Non Patent Literature 3 and Patent Literature 1 have pointed out the possibility that part of the structural defects function as active sites for the oxygen reduction reaction. Moreover, Non Patent Literature 4, Non Patent Literature 5 and Patent Literature 1 disclose that conductive carbon or the like is imparted during the constitution of electrodes.
The oxygen reduction reaction on the air electrode catalyst of a fuel cell or an air cell is a reaction involving the electron transfer from the electrode, and accordingly, in order to obtain a good oxygen reduction catalyst performance, electrons are required to rapidly move from an electrode to the vicinity of the reaction active site on the catalyst. Oxygen or proton, which is a reactant, is required to be rapidly delivered to the reaction active site. However, in general, the oxides of the group 4 and group 5 elements of transition metals described in Non Patent Literature 1 to Non Patent Literature 3 and Patent Literature 1 have insulator-like electronic states and accordingly are poor in electrical conductivity, and hardly undergo rapid reaction. Consequently, although a relatively high performance is exhibited when the cell is operated at a low electric current value, in a high electric current region, there occurs a problem that the operating voltage is lowered.
Even with the method described in Non Patent Literature 4 and Non Patent Literature 5 and Patent Literature 1, it is difficult to construct/control at a nano level an effective electron conduction path in the vicinity of the active sites, so as for the performance to remain in a low state. The introduction of a large amount of conductive carbon disturbs the supply of oxygen to the catalytic active sites; it is demanded to improve the oxygen reduction performance by allowing the imparting of electrical conductivity and the effective transport of oxygen to be compatible with each other.
For such problems, Patent Literature 2 discloses a technique to improve the electrical conductivity of the surface by introducing oxygen defects into the transition metal oxide, or by introducing oxygen defects into the transition metal oxide and by substituting part of oxygen atoms with nitrogen atoms. And, the oxygen reduction performance is improved by disposing conductive carbon in the vicinity of the structural defects to be the active sites for the oxygen reduction reaction and thus introducing good conduction paths.
Moreover, Non Patent Literature 6 discloses a technique for preparing niobium-added titanium oxide (TiO2:Nb) in which a solution obtained by dissolving NbCl5 and TiCl4 in ethanol was dropwise added to and impregnated into mesoporous C3N4, then the mesoporous C3N4 is calcined to decompose C3N4 into nitride nanoparticles, and then the nitride is oxidized to prepare the niobium-added titanium oxide (TiO2:Nb). The niobium-added titanium oxide (TiO2:Nb) is described to be useful as a platinum alternative catalyst because the niobium-added titanium oxide (TiO2:Nb) is stable and holds good electrical conductivity even after the oxidation.